The present invention relates to a laser beam scanner used in a laser printer or character reader, and more particularly, to a laser beam scanner including an improved f-.theta. lens, in which characteristics of beam-scanning a sensitive drum are improved, thereby provides an output of higher resolution.
A laser beam scanner forms an image on a recording medium through main scanning using a laser beam deflector and sub-scanning using rotation of the sensitive drum.
FIG. 1 is a schematic showing the arrangement of the optical structure of a conventional laser beam scanner.
Referring to FIG. 1, the laser beam scanner is composed of an optical source 1 for emitting a laser beam, a beam deflector 5 for deflecting the beam emitted from optical source 1 to perform the main scanning with respect to a sensitive drum 8, and an f-.theta. lens 7 for correcting errors included in the optical signal deflected from beam deflector 5. A collimating lens 4 for collimating the beam emitted from optical source 1 is positioned between optical source 1 and beam deflector 5. Also, an optical modulator 2 may be included according to the type of optical source 1 and a reflecting mirror 3 for reflecting an optical pathway may also be employed. As beam deflector 5, a rotary optical polygon 6 having multi-reflecting layers is generally adopted for performing high speed scanning.
Beam 9 is incident on rotary optical polygon 6 from a constant direction and rotary optical polygon 6 rotates in one direction, so that the direction of beam 10 scanning sensitive drum 8 after reflection from rotary optical polygon 6 is determined according to the angle formed between the reflecting surfaces of rotary optical polygon 6 and the incident beam.
If the reflecting surfaces are tilted or the shaft has shifted due to an error during the manufacture of rotary optical polygon 6, a stain caused by the scanning line is generated when an image is formed, which prevents an output of high resolution. To solve this problem, f-.theta. lens 7 and a cylindrical lens (not shown) are utilized.
The beam is incident on the reflecting surface by the cylindrical lens, thereby correcting a problem in that the position of the optical spot scanned on sensitive drum 8 experiences jitter since the reflecting surfaces of rotary optical polygon 6 are tilted. Thus, even though the beam is not incident on the reflecting surfaces in parallel due to the tilting of the reflected surface, the optical spot is always formed on the same position of the sensitive drum. In this case, when the beam is incident parallel to with the optical axis of f.theta. lens, the beam passing through the aspherical surface of the lens is not properly focused on an image forming surface. Thus, the cylindrical lens is disposed in front of the rotary optical polygon, to thereby scan the sensitive drum with the portion of the beam having a round cross-section.
The f-.theta. lens 7 which is adopted to increase the resolution controls the pathways of beams scanning the sensitive drum after deflection by beam deflector 5 at the same interval and corrects the distortion and aberration of the beam caused by any error in the manufacture of the beam deflector.
FIGS. 2A and 2B are a cross-sectional view and a perspective view, respectively, showing the f-.theta. lens of the conventional laser beam scanner, respectively.
Referring to FIGS. 2A and 2B, the structure of the conventional laser beam scanner and problems thereof will be described below.
The f-.theta. lens 7 is composed of two lenses including a first lens 7a and a second lens 7b. The beam emitting surface S of second lens 7b an aspherical surface whose curvature is different from that of spherical surface R represented as a dashed line.
It is technically very difficult to process the lens. Also, if the lens member is made of a material such as glass, there is the problem of high cost.